'\"
'\" Generated from file 'crc16\&.man' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2002, 2017, Pat Thoyts
'\"
.TH "crc16" n 1\&.1\&.4 tcllib "Cyclic Redundancy Checks"
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.SH NAME
crc16 \- Perform a 16bit Cyclic Redundancy Check
.SH SYNOPSIS
package require \fBTcl  8\&.2\fR
.sp
package require \fBcrc16  ?1\&.1\&.4?\fR
.sp
\fB::crc::crc16\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.sp
\fB::crc::crc16\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
.sp
\fB::crc::crc-ccitt\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.sp
\fB::crc::crc-ccitt\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
.sp
\fB::crc::xmodem\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.sp
\fB::crc::xmodem\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
.sp
.BE
.SH DESCRIPTION
.PP
This package provides a Tcl-only implementation of the CRC
algorithms based upon information provided at
http://www\&.microconsultants\&.com/tips/crc/crc\&.txt
There are a number of permutations available for calculating CRC
checksums and this package can handle all of them\&. Defaults are set up
for the most common cases\&.
.SH COMMANDS
.TP
\fB::crc::crc16\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.TP
\fB::crc::crc16\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
.TP
\fB::crc::crc-ccitt\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.TP
\fB::crc::crc-ccitt\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
.TP
\fB::crc::xmodem\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? \fB--\fR \fImessage\fR
.TP
\fB::crc::xmodem\fR ?-format \fIformat\fR? ?-seed \fIvalue\fR? ?-implementation \fIprocname\fR? -filename \fIfile\fR
The command takes either string data or a file name and returns a checksum
value calculated using the CRC algorithm\&. The command used sets up the
CRC polynomial, initial value and bit ordering for the desired
standard checksum calculation\&. The result is formatted
using the \fIformat\fR(n) specifier provided or as an unsigned integer
(%u) by default\&.
.sp
A number of common polynomials are in use with the CRC algorithm and
the most commonly used of these are included in this package\&. For
convenience each of these has a command alias in the crc namespace\&.
.sp
It is possible to implement the CRC-32 checksum using this crc16
package as the implementation is sufficiently generic to extend to 32
bit checksums\&. As an example this has been done already - however this
is not the fastest method to implement this algorithm in Tcl and a
separate \fBcrc32\fR package is available\&.
.PP
.SH OPTIONS
.TP
-filename \fIname\fR
Return a checksum for the file contents instead of for parameter data\&.
.TP
-format \fIstring\fR
Return the checksum using an alternative format template\&.
.TP
-seed \fIvalue\fR
Select an alternative seed value for the CRC calculation\&. The default
is 0 for the CRC16 calculation and 0xFFFF for the CCITT version\&.
This can be useful for calculating the CRC for data
structures without first converting the whole structure into a
string\&. The CRC of the previous member can be used as the seed for
calculating the CRC of the next member\&. It is also used for
accumulating a checksum from fragments of a large message (or file)
.TP
-implementation \fIprocname\fR
This hook is provided to allow users to provide their own
implementation (perhaps a C compiled extension)\&. The
procedure specfied is called with two parameters\&. The first is the
data to be checksummed and the second is the seed value\&. An
integer is expected as the result\&.
.sp
The package provides some implementations of standard CRC polynomials
for the XMODEM, CCITT and the usual CRC-16 checksum\&. For convenience,
additional commands have been provided that make use of these
implementations\&.
.TP
--
Terminate option processing\&. Please note that using the option
termination flag is important when processing data from parameters\&. If
the binary data looks like one of the options given above then the
data will be read as an option if this marker is not included\&.
Always use the \fI--\fR option termination flag before giving the data
argument\&.
.PP
.SH EXAMPLES
.PP
.CS


% crc::crc16 -- "Hello, World!"
64077

.CE
.PP
.CS


% crc::crc-ccitt -- "Hello, World!"
26586

.CE
.PP
.CS


% crc::crc16 -format 0x%X -- "Hello, World!"
0xFA4D

.CE
.PP
.CS


% crc::crc16 -file crc16\&.tcl
51675

.CE
.SH AUTHORS
Pat Thoyts
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIcrc\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH "SEE ALSO"
cksum(n), crc32(n), sum(n)
.SH KEYWORDS
checksum, cksum, crc, crc16, crc32, cyclic redundancy check, data integrity, security
.SH CATEGORY
Hashes, checksums, and encryption
.SH COPYRIGHT
.nf
Copyright (c) 2002, 2017, Pat Thoyts

.fi
